It is known to provide security enclosures with walls or sheets incorporating coils, meshes or grids of electrically responsive material and to monitor certain electrical characteristics of the material to provide an indication when the sheets containing the material have been pierced or an attempt has been made to open the enclosure through other means. A relatively simple example of such an enclosure is disclosed in PCT International Application Publication Number W087/06749, to Wolf, which relates to an alarm system particularly intended for "curtainsider" type vehicles. The system includes a flexible sheet carrying a network of closely spaced, readily frangible conductors, the alarm being operable in response to breaking a conductor upon attempting to cut or lift the sheet.
A somewhat more sophisticated security container is disclosed in UK Patent Number 1,375,926 to GAO Gesellschaft Fur Automation und Organisation mbH. The patent discloses a security container provided with a wall including one or more looped electrical conductors which are connected to a circuit which monitors current passing through the loops.
A somewhat different approach is taken in U.S. Pat. No. 4,785,743 to Dalphin, which relates to a protected room. The patent is particularly directed to electronic bank tellers and in particular the protection of the keyboard where a user supplies an access code. Such keyboards utilise an interrupter assembly and represent a weak point in the protective wall of the apparatus. The invention relates to the provision of parallel conductive sub tracks beneath the key pad. Movement activating the interrupter, as would occur from a user pressing a number on the key pad, has an equal effect on the conductivity of the two sub tracks, while an attempt at unauthorised intrusion has an unequal effect on the conductivity of the tracks. A balanced Wheatstone bridge is coupled to the sub tracks and is used to detect any unequal effect on the conductivity of the tracks.
European Patent Publication Number 277 679 to Seculock BV discloses a device for the protected storage of objects and includes a system including a layer of electrically conducting material provided with two electrodes which are connected to a processing circuit and are mounted at a distance from one another on the layer. The processing circuit monitors the resistance of the layer, which changes if any attempt is made to penetrate the layer.
However, the present invention is more closely related to the security enclosure disclosed in UK Patent Application GB2220513A, to W. L. Gore & Associates Inc. The disclosed security enclosure is formed from layers of flexible material including a matrix of diagonally extending semi-conductive lines printed on to a rectangular thin insulating film. The matrix of lines forms a continuous conductor which is broken if attempts are made to penetrate the film. The circuit is monitored by opening the conductor at one point and measuring the change of resistance between the two ends of the circuit.